High-Current PCB Traces

ABSTRACT

The present disclosure relates to systems and methods using thermal vias to increase the current-carrying capacity of conductive traces on a multilayered printed circuit board (PCB). In various embodiments, parameters associated with vias may be selected to control various electrical and thermal properties of the conductive trace. Such parameters include the via diameter, a plating thickness, a number of vias, a placement of the vias, an amount of conductive material to be added or removed from the conductive trace, a change in the resistance of the conductive trace, a change in a fusing measurement of the conductive trace, and the like.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 62/325,376, filed Apr. 20, 2016, andtitled “HIGH-CURRENT PCB TRACES,” which is incorporated herein byreference in their entireties.

TECHNICAL FIELD

The present disclosure relates to systems and methods using thermal viasto increase the current-carrying capacity of a conductive trace on amultilayered printed circuit board (PCB). More specifically, but notexclusively, the present disclosure may be implemented by creatingthermal vias through a plurality of layers of high current-carrying PCBtraces and selecting the parameters of such vias to control electricaland/or thermal characteristics of the conductive trace.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the disclosure aredescribed, including various embodiments of the disclosure withreference to the figures included in the detailed description.

FIG. 1 illustrates a thermal network of a layered PCB consistent withthe present disclosure.

FIG. 2 illustrates a trace comprising a plurality of layers andcomprising a plurality of vias disposed between the layers consistentwith the present disclosure.

FIG. 3A illustrates a diagram of a first PCB including three differentpatterns of high-current traces consistent with embodiments of thepresent disclosure.

FIG. 3B illustrates a second PCB including three different patterns ofhigh-current traces having different numbers of vias consistent withembodiments of the present disclosure.

FIG. 4 illustrates a diagram of a second prototype PCB including 11high-current traces having a varying number and density of viasconsistent with the present disclosure.

FIG. 5 illustrates variables representing a separation between adjacentvias, the diameter of the vias, and the wall thickness of the viasconsistent with embodiments of the present disclosure.

FIG. 6 illustrates variables representing a length and width of acurrent-carrying trace consistent with embodiments of the presentdisclosure.

FIG. 7 illustrates variables representing the total conductive materialin a current-carrying trace consistent with the present disclosure.

FIG. 8 illustrates a plot of an increase in conductive material as afunction of via drill diameters for a variety of minimum spacing valuesbetween vias consistent with embodiments of the present disclosure.

FIG. 9 illustrates a plot of an optimal via drill size as a function ofvia plating thickness for a plurality of minimum spacing values betweenvias consistent with embodiments of the present disclosure.

FIG. 10 illustrates a plot of an optimal via drill size versus theminimum spacing between vias for a plurality of values of via platingthicknesses consistent with embodiments of the present disclosure.

FIG. 11 illustrates a plot of a maximum via drill diameter as a functionof copper layer thickness for a plurality of via plating thicknessesconsistent with embodiments of the present disclosure.

FIG. 12 illustrates a plot of copper layer thickness as a function ofvia drill diameters and indicates regions in which conductive materialis added for various value of via plating thicknesses consistent withembodiments of the present disclosure.

FIG. 13 is a two-dimensional conceptual illustration of a hole in a PCBtrace and illustrates a flow of electrical current around the holeconsistent with embodiments of the present disclosure.

FIG. 14 is a three-dimensional conceptual illustration of a plated viaand illustrates a flow of electrical current through the plated viaconsistent with embodiments of the present disclosure.

FIG. 15 illustrates a simulation of an electrical current flow through aPCB trace and a plated via consistent with embodiments of the presentdisclosure.

FIG. 16 illustrates an area that exactly encompasses a drill hole of avia consistent with embodiments of the present disclosure.

FIG. 17 illustrates the area of FIG. 16 with a via inserted consistentwith embodiments of the present disclosure.

FIG. 18 illustrates a plurality of layers approximating a uniformcurrent injected on the edge of a plated via consistent with embodimentsof the present disclosure.

FIG. 19 illustrates a plurality of layers of a conductive trace and avia plating consistent with embodiments of the present disclosure.

FIG. 20 illustrates a simplified model of a current flowing through aplated via represented as two resistors, each of which represents halfof the plated via consistent with embodiments of the present disclosure.

FIG. 21 illustrates a representation of a via cutout including foursections that may be used to determine the resistance of the cutoutconsistent with embodiments of the present disclosure.

FIG. 22 illustrates a plot of a via drill diameter as a function of viaplating thickness and indicates regions in which electrical resistanceis reduced by the addition of vias for various value of conductivematerial, H_(c), consistent with embodiments of the present disclosure.

FIG. 23 illustrates a representation of a simulation model used toverify the accuracy of estimates presented herein for assessing thechange in electrical resistance associated with the addition of aplurality of vias to a conductive trace consistent with embodiments ofthe present disclosure.

FIG. 24 illustrates a plot of a change in resistance as a function ofvia count for two scenarios of approximated values and simulationresults consistent with embodiments of the present disclosure.

FIG. 25 illustrates a plot of a change in resistance as a function ofvia count for two scenarios of approximated values and simulationresults consistent with embodiments of the present disclosure.

FIG. 26 illustrates a module of a PCB trace comprising a plurality ofsolder-filled vias consistent with embodiments of the presentdisclosure.

FIG. 27 illustrates a model that may be used to simulate variable sizesolder-filled vias consistent with embodiments of the presentdisclosure.

FIG. 28 illustrates a change in electrical resistance as a function of avia diameter that is filled with solder consistent with embodiments ofthe present disclosure.

FIG. 29 illustrates a temperature rise of a conductive trace as afunction of the number of vias in the trace consistent with embodimentsof the present disclosure.

FIG. 30 illustrates a simulation of a multilayer current-carrying tracewith a plurality of vias that provide thermally conductive pathsconsistent with embodiments of the present disclosure.

FIG. 31 illustrates a simulation of a multilayer current-carrying tracewith a plurality of vias that provide thermally conductive pathsconsistent with embodiments of the present disclosure.

FIG. 32 illustrates a plot of a normalized fusing measurement as afunction of a number of vias in a conductive trace consistent withembodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure pertains to systems and methods for improvingvarious characteristics of high-current PCB traces using thermal vias.In various embodiments, improvement in characteristics such as currentcarrying capacity (CCC), solderability, and heat dissipation may berealized by providing a plurality of vias in the trace. In variousembodiments, vias may equalize the temperature between layers, increasethe amount of conductive material in the trace, decrease the traceresistance, and/or improve the solderability of the trace. All of theforegoing characteristics offer improvements in CCC. Additionally, thepresent disclosure presents techniques for analyzing and designingchanges in trace resistance due to vias.

A variety of characteristics and parameters of vias may be selected toachieve a desired electrical or thermal response of a conductive traceconsistent with the present disclosure. Such parameters andcharacteristics include the via diameter, a plating thickness, a numberof vias, a placement of the vias, an amount of conductive material to beadded or removed from the conductive trace, a change in the resistanceof the conductive trace, a change in a fusing measurement of theconductive trace, and the like.

The systems and methods disclosed herein may be applied in a variety ofapplications. One specific application may pertain to equipment used tomonitor and control electrical power systems. Modern electrical powerdelivery systems may be monitored, controlled, automated, and/orprotected using intelligent electronic devices (IEDs). IEDs may be usedto monitor equipment of many types, including electric transmissionlines, electric distribution lines, current transformers, busses,switches, circuit breakers, and a variety of other types of electricalpower system infrastructure. Events within electrical power systems(e.g., an overcurrent event resulting from a fault) may cause electricalspikes. Such spikes may be transferred to the IEDs monitoring affectedequipment. In some applications, protective relays may be required towithstand current flows of up to 100 times the current flow associatedwith typical operation for a period of time. In some cases, IEDs may bedesigned to monitor a nominal current of 5 amperes. During certainevents, the current may increase to 500 amperes, requiring the design ofsuch IEDs to provide a CCC of at least 500 amperes for a period of time(e.g., 1 second). This level of current flow may generate substantialheating. The resulting heat, if not dissipated sufficiently, may causethermal damage or malfunction. Various embodiments consistent with thepresent disclosure may improve heat dissipation. In addition toapplications related to equipment for monitoring electrical powersystems, the systems and methods disclosed herein may be applied in anyapplication involving high-current PCB traces.

In some embodiments, the techniques disclosed herein may also improvethe ability to solder connections into high-current traces. Large PCBtraces may be act as heat sinks at the solder points. As a result,connectors may not solder well into a large current-carrying trace. Thiscan be detrimental for the CCC of the trace, as insufficient solder maysignificantly increase resistance, heating, and otherwise the generalcurrent withstand of the trace. The techniques disclosed herein mayimprove the solderability of large PCB traces.

The CCC of a PCB trace is affected by a variety of parameters, includingthe physical dimensions of the trace, the electrical resistance of thetrace, and the ability to dissipate heat. The CCC of a PCB may beincreased by increasing the physical dimensions of the trace, andthereby increasing the amount of conductive material in the trace. Invarious applications, however, the available space may be constrained.Space constraints may be partially addressed by using a smaller trace ona plurality of layers; however, this approach may be limited by thethermal performance of the design. In addition, the flow of electricalcurrent through the trace generates heat proportional to the resistanceof the trace. Accordingly, reduction in the electrical resistance of thetrace results in reduced heat generation and reduced power loss, andtherefore, improves CCC. Smaller traces tend to exhibit higherresistance, and therefore, greater heating. The CCC of a multi-layertrace may be limited based on thermal performance.

FIG. 1 illustrates a thermal network of a layered PCB consistent withthe present disclosure. The layers may exhibit different thermalresponses owing to one or more of the differences in thermal resistanceassociated with the layer, the heat dissipation capacity of each layer,and differences in layer thickness associated with each layer. The heatdissipation capacity may be affected by various factors, includingproximity to a cooling system. In some embodiments, particular thoseused in rugged or high-dependability applications, cooling may beachieved passively (e.g., by air flow). The heat dissipation from thelowest layers may be less than the heat dissipation from the higherlevels. Accordingly, the thermal performance of the layer that has thelowest heat dissipation may limit the overall CCC. Improving thermalequalization among layers using the techniques disclosed herein mayreduce the disparity between layers, and thus, may allow for a greateroverall CCC.

In various embodiments consistent with the present disclosure, aplurality of vias may be used to reduce the thermal resistances (i.e.,R_(N-1), R_(n), R₂, R₁), between layers. As may be appreciated, reducingthe value of the thermal resistances prevents the innermost layer frombecoming significantly hotter than the other layers.

FIG. 2 illustrates a trace 200 comprising a plurality of layers 202 andcomprising a plurality of vias 206 disposed between the layers 202consistent with the present disclosure. Current ports 204 are inconnection with each layer 202 of the multilayer trace 200. The largenumber of vias 206 disposed between the layers may provide severaldesirable characteristics, including: increasing the CCC of the trace200, decreasing the resistance of the trace 200, improving thermalequalization among the plurality of layers 202, and improving thesolderability of the trace 200.

The electrical and thermal performance of trace 200 may be affected bythe size of the vias. Accordingly, a hole size may be selected indesigning trace 200 to improve the CCC of trace 200 for a specificapplication. In selecting a hole size, the following considerations maybe implicated:

-   -   1. The conductive material removed by the via drilling        procedure. The size of the hole may be selected to ensure that        conductive material added by the via is greater than, or equal        to the amount of copper removed by the drilling procedure used        to create the vias.    -   2. The effect of the hole size on the electrical resistance. As        described in greater detail below, the size of the hole impacts        the resistance of the trace. In general, the hole increases the        resistance in the plane of the trace; however, a via barrel (as        opposed to a simple hole in the plane) may provide a current        path that has a lower resistance.    -   3. The placement of the vias. In various embodiments, the vias        may be placed throughout the trace or may be concentrated around        connector ports to improve solderability due to improved heat        conduction between the layers 202.

FIG. 3A illustrates a diagram of a first PCB 300 including threedifferent patterns of high-current traces consistent with embodiments ofthe present disclosure. As illustrated, Channel C is populated with viasalong its length, and Channel A does not include any vias. Channel Bcontains vias around the solder points, which may improve solderability.The prototype comprises a 12-layer board contained within a 62 milthickness using 0.5 oz. of copper. The high-current traces areduplicated on each layer. Experimental testing of PCB 300 showed thatall of the traces passed testing of an electrical current of 500 A for 1second. Channel C exhibited the best solderability.

FIG. 3B illustrates a second PCB 350 including three different patternsof high-current traces having different numbers of vias consistent withembodiments of the present disclosure. Each channel on PCB 350 includesa different number of vias. As described above in connection with FIG.3A, the vias may be disposed around solder points to improvesolderability and disposed along a length of the channels to improve thethermal performance of PCB 350.

FIG. 4 illustrates a diagram of a second prototype PCB 400 including 11high-current traces having a varying number and density of vias andconsistent with the present disclosure. PCB 400 has the followingspecifications:

-   -   Trace length: 2.39 in. between connectors.    -   Trace width: 0.25 in.    -   Number of layers: 6.    -   Copper weight: 1 oz.    -   Total thickness of all copper layers: 10.5 mil.    -   Board Thickness: 62 mil    -   Via Diameter: 13 mil.    -   Via Plating Thickness: 1 mil typical, 0.8 mil minimum

The experimental results for PCB 400 may also be verified using theforegoing parameters in a simulation.

FIGS. 5, 6, and 7 illustrate various parameters that may be related todetermination of the change in conductive material in a conductivetrace. FIG. 5 illustrates variables representing a separation betweenadjacent vias, the diameter of the vias, and the wall thickness of thevias consistent with embodiments of the present disclosure. FIG. 6illustrates variables representing a length and width of acurrent-carrying trace consistent with embodiments of the presentdisclosure. FIG. 7 illustrates variables representing the totalconductive material, H_(c), in a current-carrying trace consistent withthe present disclosure.

As illustrated in FIG. 5, a keep-out area, A_(v), associated with eachvia may be represented as a function of the minimum space between twoadjacent vias and the diameter of the vias d_(v). For the conductivetrace shown in FIG. 6, having a width W and a length L, the maximumnumber of vias, n_(max), is given by Eq. 1.

$\begin{matrix}{n_{\max} = \frac{L \cdot W}{\left( {l_{m} + d_{v}} \right)^{2}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

Although FIG. 6 illustrates a rectangular conductive trace, theteachings of the present disclosure may be applied to conductive tracesof any geometry such as is illustrated in FIG. 3. A net change in thecopper volume of the trace may also be determined. The amount ofconductive material removed by the drilling procedure, V_(c−), may bedetermined using Eq. 2.

$\begin{matrix}{V_{c -} = {{\pi \left( \frac{d_{v}}{2} \right)}^{2} \cdot H_{c}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

After the hole is drilled, conductive material is plated on the insideof the via hole with a thickness t_(v). The plating adds additionalconductive material in the form of a hollow cylinder with a heightextending the entire thickness of the board. The added conductivematerial, V_(c+), may be calculated using Eq. 3.

Volume of Outer Cylinder Volume of Inner Cylinder

$\begin{matrix}{V_{c +} = {{{\pi \left( \frac{d_{v}}{2} \right)}^{2} \cdot H_{b}} - {{\pi \left( \frac{d_{v} - {2\; t_{v}}}{2} \right)}^{2} \cdot H_{b}}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

Eq. 3 may be alternatively expressed in Eq. 4.

$\begin{matrix}{V_{c +} = {{\pi \left\lbrack {\left( \frac{d_{v}}{2} \right)^{2} - \left( \frac{d_{v} - {2\; t_{v}}}{2} \right)^{2}} \right\rbrack} \cdot H_{b}}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

The net change in conductive material for a single via, V_(c,(1via)),may be determined using Eq. 5.

V _(c,(1via)) =V _(c+) −V _(c−)  Eq. 5

If a via results in a net increase in conductive material, it may beadvantageous in certain applications to maximize the number of vias. Thetotal conductive material that may be added for vias having a minimalspacing may be determined using Eq. 5 and Eq. 1, as shown in Eq. 6.

$\begin{matrix}{V_{c} = {V_{c,{({1\; {via}})}} \cdot {\frac{LW}{\left( {l_{m} + d_{v}} \right)^{2}}.}}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

A percent increase in the total conductive material may be determinedusing Eq. 7.

$\begin{matrix}{\frac{\% \mspace{14mu} {ChangeinCopper}}{100} = \frac{V_{c} + {WLH}_{c}}{{WLH}_{c}}} & {{Eq}.\mspace{14mu} 7}\end{matrix}$

Eq. 7 may be combined and simplified, as expressed in Eq. 8, in a formthat is not a function of the trace dimensions.

$\begin{matrix}{\frac{\% \mspace{14mu} {ChangeinCopper}}{100} = \frac{{4\; H_{c}l_{m}^{2}} - {d_{v}^{2}{H_{c}\left( {\pi - 4} \right)}} - {4\; H_{b}\pi \; t_{v}^{2}} + {4\; {d_{v}\left( {{2\; H_{c}l_{m}} + {H_{b}\pi \; t_{v}}} \right)}}}{4\; {H_{c}\left( {d_{v} + l_{m}} \right)}^{2}}} & {{Eq}.\mspace{14mu} 8}\end{matrix}$

FIG. 8 illustrates a plot of an increase in conductive material as afunction of via drill diameters for a variety of minimum spacing valuesbetween vias consistent with embodiments of the present disclosure. Inthe illustrated plot, the values of l_(m) are, from top to bottom, 5, 6,7, 8, 9, 10, 12, 14, 16, and 18 mil. For a 62 mil thick PCB with 10.5mil of combined copper across the layers, and 1 mil of via wall platingthickness, the percent of additional copper increases as the via densityincreases (i.e., as l_(m) decreases).

The value of the optimal drill size may be calculated by taking thepartial derivative of Eq. 8, which is shown in Eq. 9.

$\begin{matrix}{d_{v{({best})}} = \frac{2\; \left( {{H_{b}l_{m}t_{v}} + {2H_{b}t_{v}^{2}}} \right)}{{H_{c}l_{m}} + {2H_{b}\; t_{v}}}} & {{Eq}.\mspace{14mu} 9}\end{matrix}$

Eq. 9 may be plotted versus plating thickness for various values ofl_(m).

FIG. 9 illustrates a plot of an optimal via drill size as a function ofvia plating thickness for a plurality of minimum spacing values betweenvias consistent with embodiments of the present disclosure. In theillustrated plot, the values of l_(m) are, from top to bottom, 40, 30,20, 15, 10, and 6 mil. In various embodiments, manufacturingconsiderations or other issues (e.g., a limited number of drill sizes)may limit various design parameters, such as via diameter. Suchconstraints may be addressed in various embodiments by makingappropriate adjustments to other parameters (e.g., a plating thickness).In one specific example, a drill size of 5 mil may be desired tomaximize the increase of conductive material. See FIG. 8. If a 5 mildrill size is not available, the via plating thickness may be adjustedto place the optimal drill size within an accessible window. Withreference to FIG. 9, a plating thickness of about 1.5 mil corresponds toan optimal drill size of approximately 13 mil for maximum addition ofconductive material. Varying combinations of drill size, spacing, and/ordensity may be used to achieve a desired result (e.g., maximizing anincrease in conductive material, decreasing resistance, etc.).

FIG. 10 illustrates a plot of an optimal via drill size versus theminimum spacing between vias for a plurality of values of via platingthicknesses, t_(v), consistent with embodiments of the presentdisclosure.

FIG. 11 illustrates a plot of a maximum via drill diameter as a functionof copper layer thickness for a plurality of via plating thicknesses,t_(v), consistent with embodiments of the present disclosure. In theillustrated plot, the values of t_(v) are, from top to bottom, 1.2, 1.0,and 0.8 mil.

As the size of the drill increases above the optimal value, more copperis removed by the drilling process than is added by the plating process,as shown in FIG. 8. Further, the limits of via drill size for a givenset of geometric conditions may be determined using Equation 5. Theresults of setting Equation 5 greater than zero and solving for d_(v),are shown in Eqs. 10-12. Eq. 11 is plotted in FIG. 12.

$\begin{matrix}{V_{c,{({1\; {via}})}} > 0} & {{Eq}.\mspace{14mu} 10} \\{d_{v} < {\frac{2H_{b}\; t_{v}}{H_{c}} + {2\sqrt{- \frac{{{- H_{b}^{2}}t_{v}^{2}} + {H_{b}H_{c}t_{v}^{2}}}{H_{c}^{2}}}}}} & {{Eq}.\mspace{14mu} 11} \\{d_{v} > {\frac{2H_{b}\; t_{v}}{H_{c}} - {2\sqrt{\frac{{{- H_{b}^{2}}t_{v}^{2}} + {H_{b}H_{c}t_{v}^{2}}}{H_{c}^{2}}}}}} & {{Eq}.\mspace{14mu} 12}\end{matrix}$

FIG. 12 illustrates a plot of copper layer thickness, H_(c), as afunction of via drill diameters and indicates regions in whichconductive material is added for various value of via platingthicknesses, t_(v), consistent with embodiments of the presentdisclosure. In various embodiments consistent with the presentdisclosure, FIG. 12 may provide a reference for determining whether aproposed via drill diameter and thickness of a conductive layer resultin an increase or a decrease in the conductive material in a conductivetrace as a result of the addition of vias.

FIG. 13 is a two-dimensional conceptual illustration of a hole in a PCBtrace and illustrates a flow of electrical current around the holeconsistent with embodiments of the present disclosure. When the hole isdrilled, current on the trace layers is forced to flow around the hole,and thus, increasing electrical resistance. In order to offset, or evenreduce the increase in electrical resistance, additional conductivematerial may be added by increasing the plating in a via.

FIG. 14 is a three-dimensional conceptual illustration of a plated viaand illustrates a flow of electrical current through the plated viaconsistent with embodiments of the present disclosure. As illustrated inFIG. 14, the plating within the via creates a three-dimensional paththrough which the current can flow. Provided the via wall is formed of amaterial with a sufficiently low resistance, the resistance may decreasein comparison to a trace without a via.

FIG. 15 illustrates a simulation of an electrical current flow through aPCB trace 300 and a plated via 302 consistent with embodiments of thepresent disclosure. The magnitude and direction of the current flow atvarious points is indicated by a plurality of arrows. As illustrated, aportion of the current flows through the via plating. The current flowthrough the via plating decreases with distance from the PCB trace.

FIGS. 16, 17, 18, and 19 illustrate a conceptual model of a plated viaconsistent with embodiments of the present disclosure. FIG. 16illustrates an area that exactly encompasses a drill hole of a via. Aninitial step may be to analyze the resistance of the area. FIG. 17illustrates the area of FIG. 16 with a via inserted. When a via isinstalled, the via hole first consumes the entire length and width ofthe box, but the via plating then reduces the hole size as shown in FIG.17. The next step may be to analyze the resistance of the area after thevia is inserted.

FIG. 18 illustrates a plurality of layers approximating a uniformcurrent injected on the edge of a plated via consistent with embodimentsof the present disclosure. Certain simplifying assumptions may be madeto reduce the complexity of the geometry of the via. As illustrated, thecurrent is injected into a via at multiple, yet finite points atjunctions with the layers, as shown in FIG. 18. Using a large number oflayers, or using a small distance between them, the model may besimplified by assuming either an infinite number of layers, or aninfinitesimal distance between the layers. In other words, the model mayassume that the current is injected uniformly onto the via, as shown inthe simplified representation in FIG. 18.

FIG. 19 illustrates a plurality of layers of a conductive trace and avia plating consistent with embodiments of the present disclosure. InFIG. 19, the plurality of layers may be combined, and it may be assumedthat the current approaches the via plating and cutout uniformly.Notably, the simulation results in FIG. 15 show a divergence from thisassumption. Finally, it may be assumed that the resistance of the viaplating and the resistance of the trace are a parallel combination oftwo resistors, R_(v) and R_(t).

FIG. 20 illustrates a simplified model of a current flowing through aplated via represented as two resistors, each of which represents halfof the plated via consistent with embodiments of the present disclosure.The approximate resistance of each semi-cylinder may be representedusing Eq. 13.

$\begin{matrix}{R_{A} = {R_{B} = \frac{\rho_{c}\pi \; d_{v}}{2\; t_{v}H_{c}}}} & {{Eq}.\mspace{14mu} 13}\end{matrix}$

The equivalent resistance of R_(A) and R_(B) is expressed in Eq. 14.

$\begin{matrix}{R_{v} = {{R_{A}{}R_{b}} = {{\frac{1}{2}R_{A}} = \frac{\rho_{c}\pi \; d_{v}}{4t_{v}H_{c}}}}} & {{Eq}.\mspace{14mu} 14}\end{matrix}$

FIG. 21 illustrates a representation of a via cutout including foursections that may be used to determine the resistance of the cutoutconsistent with embodiments of the present disclosure. As illustrated,the cutout is divided into four regions, namely R_(A), R_(B1), R_(B2),and R_(C). The resistance of R_(A) and R_(C) may be determined using Eq.15.

$\begin{matrix}{R_{A} = {R_{C} = \frac{\rho_{c}\; t_{v}}{H_{c}d_{v}}}} & {{Eq}.\mspace{14mu} 15}\end{matrix}$

The resistance R_(B1) and R_(B2) may be determined as an integralbecause of the varying cross-sectional area. Specifically, the circle ofradius may be substrated from the height of the region. The width of thesection at any given position, x may be determined using Eq. 16.

$\begin{matrix}{w = {\frac{d_{v}}{2} - \sqrt{\left( \frac{d_{v} - {2\; t_{v}}}{2} \right)^{2} - x^{2}}}} & {{Eq}.\mspace{14mu} 16}\end{matrix}$

Since R_(B1)=R_(B2) and these sections are in parallel, the effectiveresistance is simply half of R_(B1). Further, R_(B1) may be divided intotwo equal sections (i.e., form x=0 to

$x = {\frac{d_{v}}{2} - {t_{v}.}}$

Thus, the resistance of only the shaded region in FIG. 21 may becalculated, using Eq. 17.

$\begin{matrix}{R_{B} = {\int_{x}^{\frac{d_{v}}{2}\; - t_{v}}{\frac{\rho_{c}}{H_{c}\left( {\frac{d_{v}}{2} - \sqrt{\left( \frac{d_{v} - {2t_{v}}}{2} \right)^{2} - x^{2}}} \right)}\ {dx}}}} & {{Eq}.\mspace{14mu} 17}\end{matrix}$

The resistance of the entire cutout may be calculated using Eq. 18.

$\begin{matrix}{R_{t} = {\frac{\rho_{c}t_{v}}{H_{c}d_{v}} + {\int_{x}^{\frac{d_{v}}{2}\; - t_{v}}{\frac{\rho_{c}}{H_{c}\left( {\frac{d_{v}}{2} - \sqrt{\left( \frac{d_{v} - {2t_{v}}}{2} \right)^{2} - x^{2}}} \right)}\ {dx}}}}} & {{Eq}.\mspace{14mu} 18}\end{matrix}$

Finally, since R_(t) is in parallel with the via wall R_(v), theresistance of the entire via structure may be calculated using Eq. 19.

$\begin{matrix}{R_{via} = \frac{1}{\begin{matrix}{\frac{\rho_{c}t_{v}}{H_{c}d_{v}} + {\int_{0}^{\frac{d_{v}}{2}\; - t_{v}}{\frac{\rho_{c}}{H_{c}\left( {\frac{d_{v}}{2} - \sqrt{\left( \frac{d_{v} - {2t_{v}}}{2} \right)^{2} - x^{2}}} \right)}\ {dx}}} +} \\\frac{4t_{v}H_{c}}{\rho_{c}\pi \; d_{v}}\end{matrix}}} & {{Eq}.\mspace{14mu} 19}\end{matrix}$

The foregoing analysis may be used to determine an approximate traceresistance including a plurality of vias. The resistance of the squareshown in FIG. 16 may be defined using Eq. 20.

$\begin{matrix}{R_{box} = {\frac{\rho_{c}d_{v}}{H_{c}d_{v}} = \frac{\rho_{c}}{H_{c}}}} & {{Eq}.\mspace{14mu} 20}\end{matrix}$

A ratio of via resistance to electrical resistance of the original tracemay be defined using Eq. 21.

$\begin{matrix}{\alpha = \frac{R_{via}}{\frac{\rho_{c}}{H_{c}}}} & {{Eq}.\mspace{14mu} 21}\end{matrix}$

Combining and simplifying Eq. 21 with Eq. 19 yields Eq. 22.

$\begin{matrix}{\alpha = \frac{H_{c}}{\frac{1}{\begin{matrix}{\frac{\rho_{c}t_{v}}{H_{c}d_{v}} + {\int_{0}^{\frac{d_{v}}{2}\; - t_{v}}{\frac{\rho_{c}}{H_{c}\left( {\frac{d_{v}}{2} - \sqrt{\left( \frac{d_{v} - {2t_{v}}}{2} \right)^{2} - x^{2}}} \right)}\ {dx}}} +} \\\frac{4t_{v}H_{c}}{\rho_{c}\pi \; d_{v}}\end{matrix}}}} & {{Eq}.\mspace{14mu} 22}\end{matrix}$

Eq. 22 represents a percent change in resistance when a via is inserted.

FIG. 22 illustrates a plot of a via drill diameter as a function of viaplating thickness and indicates regions in which electrical resistanceis reduced by the addition of vias for various value of conductivematerial, H_(c), consistent with embodiments of the present disclosure.FIG. 22 also identifies regions in which electrical resistance isincreased as a result of vias.

A change in trace resistance may be estimated for a given number of viasinserted. For a trace of length L and width W, the entire trace may bedivided into n boxes having an dimension d_(v)×d_(v). The number ofboxes in the trace may be determined using Eq. 23.

$\begin{matrix}{n_{boxes} = \frac{LW}{d_{v}^{2}}} & {{Eq}.\mspace{14mu} 23}\end{matrix}$

The resistance of the trace may be determined using Eq. 24.

$\begin{matrix}{R_{trace} = \frac{\rho_{c}L}{H_{c}W}} & {{Eq}.\mspace{14mu} 24}\end{matrix}$

When a via is punched into the trace, the added via may change theresistance of an associated box by a factor α; however, the resistanceof the entire trace does not change by a unless a via is added to everybox. As such, Eq. 25 and Eq. 26 may describe boundary conditions of thisapproximation.

$\begin{matrix}{{R_{trace}\left( {n = 0} \right)} = \frac{\rho_{c}L}{H_{c}W}} & {{Eq}.\mspace{14mu} 25} \\{{R_{trace}\left( {n = \frac{LW}{d_{v}^{2}}} \right)} = {\alpha {\frac{\rho_{c}L}{H_{c}W}.}}} & {{Eq}.\mspace{14mu} 26}\end{matrix}$

Eq. 27 satisfies both conditions with a linear interpolation between.

$\begin{matrix}{{R_{trace}(n)} = {\frac{\rho_{c}L}{H_{c}W}\left( {1 - {\left( {1 - \alpha} \right)\frac{{nd}_{v}^{2}}{LW}}} \right)}} & {{Eq}.\mspace{14mu} 27}\end{matrix}$

The trace resistance is a function of the number of vias in the trace aswell as of α. Instead of the via removing conductive material, which maybe replaced with more or less than has been removed by plating, thetraces may be connected through the via with solder, and the solder maybehave as a thick via wall.

FIG. 23 illustrates a representation of a simulation model used toverify the accuracy of estimates presented herein for assessing thechange in electrical resistance associated with the addition of aplurality of vias to a conductive trace consistent with embodiments ofthe present disclosure. In a simulation, currents of 250 A were injectedinto each of the two connections on the trace, for a total of 500 A. Thesimulation calculated the static loss power. The resistance wascalculated based on the current.

FIG. 24 illustrates a plot of a change in resistance as a function ofvia count for two scenarios of approximated values and simulationresults consistent with embodiments of the present disclosure. Theaccuracy of the estimates reflected in Eq. 27 may be verified using thesimulation model shown in FIG. 23, using two alpha (a) factors, as shownin Eq. 28 and Eq. 29.

α(d _(v)=13 mil,t _(v)=1 mil)=1.2945  Eq. 28

α(d _(v)=8 mil,t _(v)=1 mil)=0.827357  Eq. 29

While there is some difference between the simulation points and theapproximation, the direction of change in resistance is consistent.

FIG. 25 illustrates a plot of a change in resistance as a function ofvia count for two scenarios of approximated values and simulationresults consistent with embodiments of the present disclosure. A greaterdecrease in resistance may be achieved by increasing the wall thickness,t_(v), to 2.54 mil. The results in FIG. 25 show the results of thesimulation model shown in FIG. 23, and two alpha (α) factors, as shownin Eq. 30 and Eq. 31.

α(d _(v)=13 mil,t _(v)=2.54 mil)=0.827357  Eq. 30

α(d _(v)=8 mil,t _(v)=2.54 mil)=0.3508435  Eq. 31

The previous analysis assumes the vias are filled with air. This is inmany respects a “worst case” analysis since at least some vias willtypically be filled with solder. If solder-filling is taken intoaccount, the method of populating a trace with many vias becomes evenmore effective at improving CCC.

FIG. 26 illustrates a module of a PCB trace comprising a plurality ofsolder-filled vias consistent with embodiments of the presentdisclosure. FIG. 27 illustrates a model that may be used to simulatevariable size solder-filled vias consistent with embodiments of thepresent disclosure. It may be difficult to ensure solder-filled viasusing typical PCB manufacturing techniques. Accordingly, an analysis ofair-filled vias may be considered.

FIG. 28 illustrates a change in electrical resistance as a function of avia diameter that is filled with solder consistent with embodiments ofthe present disclosure. While hollow vias generally have worseperformance (higher resistance and less copper volume) for larger holesizes, solid vias perform significantly better as the via diameterincreases. If there is some confidence in solder filling, a designer mayopt to use larger vias to decrease the electrical resistance.

Adding vias to a conductive trace may alter the electrical properties ofthe trace by changing the amount of conductive materials in the traceand by changing the electrical resistance of the trace. These twoeffects may improve the thermal behavior of a conductive trace. Athermal analysis may begin by calculating the temperature rise in theconductive element using Eq. 32. In this example, the conductive elementis copper.

$\begin{matrix}{{\Delta \; T} = \frac{P_{D}}{M_{c}C}} & {{Eq}.\mspace{14mu} 32}\end{matrix}$

In Eq. 32, C is the heat capacity of copper, M_(c) is the mass of copperin the trace, and P_(D) is the power dissipated.

To begin, we calculate the temperature rise. The change in temperaturemay also be determined using Eq. 33.

$\begin{matrix}{{\Delta \; T} = \frac{I^{2}R_{trace}}{d_{c}V_{t}C}} & {{Eq}.\mspace{14mu} 33}\end{matrix}$

In Eq. 33, d_(c) is the density of copper and V_(t) is the volume ofcopper in the trace. The resistance of the trace is provided in Eq. 27.Substituting Eq. 27 into Eq. 33 yields Eq. 34.

$\begin{matrix}{{\Delta \; T} = \frac{I^{2}\left\lbrack {\frac{\rho_{c}L}{H_{c}W}\left( {1 - {\left( {1 - \alpha} \right)\frac{{nd}_{v}^{2}}{LW}}} \right)} \right\rbrack}{d_{c}V_{t}C}} & {{Eq}.\mspace{14mu} 34}\end{matrix}$

The term V_(t) in Eq. 34 may be substituted with an expressionrepresenting the volume of copper (i.e., the product of the length (L),width (W), conductive material (H_(c)), the trace, the net change inconductive material for a single via, V_(c,(1via)), and the number ofvias (n), as shown in Eq. 35.

$\begin{matrix}{{\Delta \; T} = \frac{I^{2}\left\lbrack {\frac{\rho_{c}L}{H_{c}W}\left( {1 - {\left( {1 - \alpha} \right)\frac{{nd}_{v}^{2}}{LW}}} \right)} \right\rbrack}{{d_{c}\left( {LWH}_{c} \right)}V_{c,{({1{via}})}}n\; C}} & {{Eq}.\mspace{14mu} 35}\end{matrix}$

Then Equation 5 can be substituted for V_(c,(1via)) in Eq. 35.Calculating the temperature rise using the trace parameters of the protoboard of FIG. 4 gives FIG. 30.

FIG. 29 illustrates a temperature rise of a conductive trace as afunction of the number of vias in the trace consistent with embodimentsof the present disclosure. As shown in FIG. 24, the electricalresistance of the trace increases for a 13 mil via; however, as shown inFIG. 30, the overall temp rise of the trace rise is reduced. For 8 milvias, the reduction in temperature is improved. The addition of 16 milvias, on the other hand, increases resistance and the temperature rise.

FIGS. 30 and 31 illustrate simulations of a multilayer current-carryingtrace with a plurality of vias that provide thermally conductive pathsconsistent with embodiments of the present disclosure. The direction andmagnitude of heat flow is illustrated with arrows, which indicate thatheat flows away from the inner-most layer to the outer-most layers.

FIG. 32 illustrates a plot of a normalized fusing measurement as afunction of a number of vias in a conductive trace consistent withembodiments of the present disclosure. The illustrated results wereobtained by inducing large currents in electrical traces having varyingnumbers of vias and measuring the voltage across the trace. The testscenario illustrated in FIG. 32 used dimensions where electricalresistance increases as more vias are added. The results show thatadding vias up to a certain threshold provides thermal benefitsresulting in an improved fusing rating. The addition of vias beyond thethreshold increases the electrical heating and therefore negates thethermal benefits provided by the vias. The illustrated results depictair-filled vias. If the vias are filled with solder, the resistance ofthese vias decreases, providing both improved thermal performance anddecreased electrical resistance. Fusing occurs when the current sourcereaches a compliance voltage limit.

While specific embodiments and applications of the disclosure have beenillustrated and described, it is to be understood that the disclosure isnot limited to the precise configurations and components disclosedherein. Accordingly, many changes may be made to the details of theabove-described embodiments without departing from the underlyingprinciples of this disclosure. The scope of the present inventionshould, therefore, be determined only by the following claims.

What is claimed is:
 1. A method for improving the current-carryingcapacity of a multilayer conductive trace on a printed circuit board,the method comprising: selecting a via diameter; selecting a platingthickness; selecting a spacing between vias; selecting a number of viasto control: an electrical resistance of the conductive trace, and athermal response of the conductive trace; fabricating the multilayerconductive trace on the printed circuit board.
 2. The method of claim 1,further comprising: estimating the electrical resistance of theconductive trace based on the via diameter, the plating thickness, thespacing between vias, and the board and conductive trace dimensions. 3.The method of claim 2, wherein estimating the electrical resistance ofthe conductive trace comprises one of simulating and calculating theelectrical resistance.
 4. The method of claim 1, further comprising:estimating the thermal response of the conductive trace based on the viadiameter, the plating thickness, and the spacing between vias.
 5. Themethod of claim 1, further comprising selecting the number of viasplaced on the conductive trace to control a quantity of conductivematerial added to the conductive trace.
 6. The method of claim 1,further comprising selecting the via diameter and the plating thicknessto increase the current-carrying capacity of the conductive trace. 7.The method of claim 1, further comprising selecting the via diameter andthe plating thickness to maximize a fusing measurement of the conductivetrace.
 8. The method of claim 1, further comprising selecting the viadiameter and the plating thickness to reduce the electrical resistanceof the conductive trace.
 9. The method of claim 1, wherein the number ofvias is selected to maximize a fusing measurement of the conductivetrace.
 10. The method of claim 1, wherein the number of vias is selectedto reduce the electrical resistance of the conductive trace.
 11. Themethod of claim 1, wherein the number of vias is selected to increasethe conductive material in the conductive trace.
 12. The method of claim1, further comprising placing the vias in proximity to a current port toimprove solderability of the conductive trace.
 13. The method of claim1, wherein fabricating the multilayer conductive trace on the printedcircuit board further comprises filling at least a portion of theplurality of vias with solder.
 14. A multilayer conductive trace on aprinted circuit board manufactured by the process of claim
 1. 15. Amultilayer printed circuit board comprising: a current-carryingconductive trace disposed on a plurality of layers in the multilayercircuit; a current port in communication with the current-carryingconductive trace; a plurality of thermal vias in thermal communicationwith the conductive trace and extending through at least a portion ofthe plurality of layers, the plurality of thermal vias configured tocontrol: an electrical resistance of the conductive trace, and a thermalresponse of the conductive trace.
 16. The multilayer printed circuitboard of claim 15, wherein the plurality of thermal vias are configuredto decrease heating of the conductive trace caused by electrical currentflow.
 17. The multilayer printed circuit board of claim 15, wherein theplurality of thermal vias comprise solder-filled vias.
 18. Themultilayer printed circuit board of claim 15, wherein the thermal viaseach comprise a diameter and a plating thickness selected to add aquantity of conductive material to the conductive trace.
 19. Themultilayer printed circuit board of claim 15, wherein the number of viasmaximizes a fusing measurement of the conductive trace.
 20. Themultilayer printed circuit board of claim 15, wherein the number of viasdecreases the electrical resistance of the conductive trace.
 21. Themultilayer printed circuit board of claim 15, wherein the vias areplaced in proximity to the current port to improve solderability of theconductive trace.